Re: [PATCH] loongarch: Only select HAVE_OBJTOOL and allow ORC unwinder if the inline assembler supports R_LARCH_{32,64}_PCREL

[Jinyang He]

This is a multi-part message in MIME format.
On 2024-06-05 23:13, Xi Ruoyao wrote:

> On Wed, 2024-06-05 at 21:18 +0800, Xi Ruoyao wrote:
> > On Wed, 2024-06-05 at 18:57 +0800, Xi Ruoyao wrote:
> > > On Tue, 2024-06-04 at 23:25 -0700, Nathan Chancellor wrote:
> > > > On Wed, Jun 05, 2024 at 01:54:24PM +0800, Xi Ruoyao wrote:
> > > > > On Tue, 2024-06-04 at 22:43 -0700, Nathan Chancellor wrote:
> > > > > > For what it's worth, I have noticed some warnings with clang that I
> > > > > > don't see with GCC but I only filed an issue on our GitHub and never
> > > > > > followed up on the mailing list, so sorry about that.
> > > > > >
> > > > > > https://github.com/ClangBuiltLinux/linux/issues/2024
> > > > > >
> > > > > > Might be tangential to this patch though but I felt it was worth
> > > > > > mentioning.
> > > > > The warnings in GCC build is definitely the issue handled by this patch.
> > > > > But the warnings in Clang build should be a different issue.  Can you
> > > > > attach the kernel/events/core.o file from the Clang build for analysis?
> > > > > I guess we need to disable more optimization...
> > > > Sure thing. Let me know if there are any issues with the attachment.
> > > Thanks!  I've simplified it and now even...
> > >
> > > .global test
> > > .type test,@function
> > > test:
> > >
> > > addi.d	$sp,$sp,-448
> > > st.d	$ra,$sp,440
> > > st.d	$fp,$sp,432
> > > addi.d	$fp,$sp,448
> > >
> > > # do something
> > >
> > > addi.d	$sp,$fp,-448
> > > ld.d	$fp,$sp,432
> > > ld.d	$ra,$sp,440
> > > addi.d	$sp,$sp,448
> > > ret
> > >
> > > .size test,.-test
> > >
> > > is enough to trigger a objtool warning:
> > >
> > > /home/xry111/t1.o: warning: objtool: test+0x20: return with modified stack frame
> > >
> > > And to me this warning is bogus?
> > Minimal C reproducer:
> >
> > struct x { _Alignas(64) char buf[128]; };
> >
> > void f(struct x *p);
> > void g()
> > {
> > 	struct x x = { .buf = "1145141919810" };
> > 	f(&x);
> > }
> >
> > Then objtool is unhappy to the object file produced with "clang -c -O2"
> > from this translation unit:
> >
> > /home/xry111/t2.o: warning: objtool: g+0x50: return with modified stack frame
> >
> > It seems CFI_BP has a very specific semantic in objtool and Clang does
> > not operates $fp in the expected way.  I'm not sure about my conclusion
> > though.  Maybe Peter can explain it better.
> Another example: some simple rust code:
>
> extern { fn f(x: &i64) -> i64; }
>
> #[no_mangle]
> fn g() -> i64 {
>      let x = 114514;
>      unsafe {f(&x)}
> }
>
> It's just lucky GCC doesn't use $fp as the frame pointer unless there's
> some stupid code (VLA etc) thus the issue was not detected.
I think this because we did not add arch special handle in 
update_cfi_state().
In our 419 repo this func has been renamed arch_update_insn_state (, now it
should be arch_update_cfi_state) and give some actions to deal with the
frame pointer case. If we support it we may deal with some case but for 
clang...

.global test
.type test,@function
test:

addi.d  $sp,$sp,-448
st.d    $ra,$sp,440
st.d    $fp,$sp,432
addi.d  $fp,$sp,448

# do something  <- not change $sp

# addi.d        $sp,$fp,-448  <- not restore sp from cfa(fp)
ld.d    $fp,$sp,432
ld.d    $ra,$sp,440
addi.d  $sp,$sp,448
ret

.size test,.-test

This will cause warning, too. (Gcc should be ok, I don't test.)

source.c: (clang 19, based 0c1c0d53931, -g)
void *foo() { return __builtin_frame_address(0); }

asm.s:
        .cfi_sections .debug_frame
        .cfi_startproc
# %bb.0:
        addi.d  $sp, $sp, -16
        .cfi_def_cfa_offset 16
        st.d    $ra, $sp, 8                     # 8-byte Folded Spill
        st.d    $fp, $sp, 0                     # 8-byte Folded Spill
        .cfi_offset 1, -8
        .cfi_offset 22, -16
        addi.d  $fp, $sp, 16
        .cfi_def_cfa 22, 0                    <- define cfa is $r22
.Ltmp0:
        .loc    0 1 22 prologue_end             # hello.c:1:22
        move    $a0, $fp
        ld.d    $fp, $sp, 0                     # 8-byte Folded Reload  
<- restore regs from non-cfa ?
        ld.d    $ra, $sp, 8                     # 8-byte Folded Reload  
<- restore regs from non-cfa ?
        .loc    0 1 15 epilogue_begin is_stmt 0 # hello.c:1:15
        addi.d  $sp, $sp, 16                                            
<- restore prev-sp from non-cfa ?
        ret

Maybe Clang have anything wrong?


Attach: tmp fix by support arch_update_insn_state.


Thanks,

Jinyang